In order to tailor certain security checks to the caller that is
attempting to load a particular piece of content, we need to be able to
attach an appropriate triggering principal to the corresponding requests.
Since most HTML content is loaded based on attribute values, this means
capturing the subject principal of the caller who sets those attributes,
which in turn means we need to make it available to `AfterSetAttr` hooks
on all relevant element types.
According to the original SVG spec, the SVG element in documents had to follow
special rules as to what was allowed for width/height attributes. This is no
longer the case, so we can decouple it from whatever internal units SVG/XML
language uses and allow all units like every other element.
Since these are just interpreted comments, there's 0 impact on actual code.
This removes all lines that match /* vim: set(.*)tw=80: */ with S&R -- there are
a few others scattered around which will be removed manually in a second part.
* Remove the generic attr preparsing mechanism from BeforeSetAttr and just preparse class attributes directly in the one place that needs to do it
* Move calls to BeforeSetAttr to after AttributeWillChange
* Remove UpdateState calls in BeforeSetAttr
* Move calls to AfterSetAttr to before UpdateState when manipulating attributes
* Remove UpdateState calls from AfterSetAttr, since they are no longer needed there
Tag #1375
